1. Field of the Invention
The present invention relates to a centrifugal switch device, and more particularly to a centrifugal switch device for an induction motor.
2. Description of the Related Art
A common used induction motor usually includes a start circuit for starting the rotation of the motor. The start circuit includes a centrifugal switch for controlling the starting and stopping of the motor, and a capacitor for providing a higher starting force so as to increase the rotational speed of the motor to reach a higher value in a short time. When the rotational speed of the motor reaches 75% of that of its full load, the centrifugal switch is used to cut of connection between the starting motor and the motor so that the rotational speed of the motor will not be increased excessively, and so that the motor is operated at a predetermined normal rotational speed.
A conventional centrifugal switch for an induction motor in accordance with the prior art shown in FIGS. 1 and 2 comprises a switch contact 8 mounted on a casing 1 of the motor and connecting to a start circuit (not shown) for controlling the rotation of the motor, an elastic strip 7 detachably contacting with the switch contact 8, a sliding member 6 mounted on a shaft (not shown) of the motor and having a distal end pressing a distal end of the elastic strip 7, two springs 9 each mounted between the sliding member 6 and the casing 1, a fan 2 secured on the shaft to rotate therewith, a pressing disk 3 secured in the fan 2 to rotate therewith and slidably mounted on the shaft for urging the sliding member 6, two urging members 5 pivotally mounted on the fan 2 and each attached to the pressing disk 3 for moving the pressing disk 3 on the shaft relative to the sliding member 6, and two springs 4 each mounted on the two urging members 5 for pivoting the two urging members 5.
In operation, the pressing disk 3 is initially pushed toward the sliding member 6 by the two urging members 5 and the two springs 4 to move the sliding member 6 toward the casing 1 to press the elastic strip 7 which then touches the switch contact 8 as shown in FIG. 1 so that the centrifugal switch device can be disposed in a state of "ON", thereby allowing the induction motor electrically connecting to a start circuit (not shown) so as to start the induction motor.
When the rotational speed of the induction motor is increased to reach 75% of that of its full load, the urging members 5 is moved from the position as shown in FIG. 1 to the position as shown in FIG. 2 by means of the centrifugal force generated by the high speed rotation of the shaft to move the pressing disk 3 away from the sliding member 6 so that the sliding member 6 is moved toward the pressing disk 3 by the restoring force of the springs 9 so as to move the sliding member 6 from the position as shown in FIG. 1 to the position as shown in FIG. 2, thereby detaching the switch contact 8 from the elastic strip 7 so that the centrifugal switch device is disposed in a state of "OFF", and the start circuit is disconnected. In such a manner, the rotational speed of the induction motor will not be increased due to the power supplied from the centrifugal switch device is cut off so that the induction motor is rotated at a normal speed.
However, heat is easily created by high friction while the sliding member 6, the elastic strip 7 and the switch contact 8 are in contact with each other during the operation. In addition, the switch is not provided with a registering apparatus for regulating the parts such that they have a tendency of sliding out of position. Further, the parts are easily attached by dirt or dust. Accordingly, the disadvantages will result in poor connection of electricity and destruction of the parts of the motor, thereby increasing difficulties of operation and costs of maintenance.
The closest prior arts of which the applicant is aware are disclosed in U.S. Pat. No. 5,602,436 to Sherman et al.; U.S. Pat. No. 2,555,703 to Restemeier; and U.S. Pat. No. 4,272,660 to Mayer et al.
In the Sherman reference, it discloses a shaft 9, a flange 25, a sleeve 17, a table 30, a contact 27, a switch 28, a spring 34, and a weight 31. The table 30 engages the contact 27 of the switch 28, and the spring 34 biases the table 30 normally away from the contact 27 toward the flange 25 so that the switch 28 is not normally closed. When the rotational rate of the shaft 9 reaches a predetermined speed, the weights 31 pivot outward to move the flange 25 along the sleeve 17 so as to engage the table 30. The table 30 is then urged downward to depress the contact 27 and actuate the switch 28 to disengage the starting winding so that the switch 28 is actuated at an appropriate speed.
In the Restemeier reference, it discloses a disc 24 which is embossed to provide an annular rim portion 30 which is adapted to engage a button 31 forming a part of a microswitch 32. When the motor is idle, the coil spring 29 insures that the rim 30 of the disc 24 will be in contact with the button 31 to hold the electrical contacts in engagement with the microswitch, thereby enabling the motor to start rotating. As the speed of the motor increases, the arms 20 and 22 are urged in a radially outward direction by the centrifugal force, and this outward movement of the arms 20 and 22 is translated into axial movement of the disc 24 so that the button 31 is released, the contacts associated with the microswitch are opened, and flow of current to the starting winding of the motor is thereby stopped. However, the disc 24 is secured on a sleeve 27 which is slidably secured on the shaft 17 of the motor to rotate herewith. Therefore, the disc 24 is rotated with the shaft 17 so that the rim portion 30 constantly and continuously contacts with the button 31 during the rotation of the shaft 17, thereby easily creating high heat due to friction, and thereby easily wearing the button 31.